Low-temperature thermo-tectonic evolution of the Altai-Sayan mountains, South Siberia, Russia

The Altai-Sayan (AS) mountains are part of the vast intracontinental Central Asian Deformation Zone (CADZ), stretching out in front of the Himalayan-Tibetan orogenic system. The Russian (South Siberian) part of the AS mountains is composed of tectonic terranes (e.g. Gorny Altai, West Sayan) which collided with the Siberian craton during the Late Paleozoic evolution of the Paleo-Asian Ocean. Strain and stresses induced by the ongoing convergence of the Indian and Eurasian plates are to some extent partitioned as a reactivation of pre-existing structures in Eurasia’s interior, and largely contributed to the development of the CADZ. The AS mountains are located in the NW section of the CADZ and have been transpressionally reactivated in the this tectonic framework since the Pliocene. Structural data of the present authors (Dehandschutter et al., 2001) show that reactivation is still in progress, and is mainly manifested along older structures. Many of these structures appear to be Late Paleozoic strike-slip faults created during the accretion and assembly of the Siberian continent (Buslov et al., 2003), and in a broader sense, of the Pangean supercontinent. To obtain more detailed insights in the low-temperature thermo-tectonic evolution of the region mica and amphibole 40Ar/39Ar and apatite fission-track (AFT) data were collected. The 40Ar/39Ar data are confined to granitoid-gneissic rocks from the Teletskoye and Chuya-Kurai region. Most Ar-ages ages fall between 380 Ma and 350 Ma, corresponding with the Late Devonian-Early Carboniferous collision, while a single sample (260 Ma) dates the Permian collision. AFT ages were obtained from a more extensive area, i.e. the Teletskoye graben basement (30 samples, including 2 elevation profiles), The Chuya-Kurai basin and ranges (3 samples), the Chulyshman Plateau (9), the Dzhulukul basin (4), and western Gorny Altai (5). Cretaceous AFT ages were found for most samples. Length data (mean lengths ~11.5 to 13.5 µm and negatively skewed distributions) point to a substantial thermal effect reflecting a moderate to strong Apatite Partial Annealing Zone signature. AFT modelling of the data shows a consistent Meso-Cenozoic three-phased cooling history. A Late Jurassic-Cretaceous cooling affected the investigated AS region (De Grave and Van den haute, 2002) and is possibly related to denudation associated with tectonic activity in the West Siberian Basin, west of the study area and the Mongol-Okhotsk orogeny to the east. An ubiquitous Late Cretaceous-Paleogene peneplanation in the region is reflected by a period of stability in our modelled cooling paths. A Pliocene cooling pulse generated by the model is interpreted to reflect denudation induced by the Pliocene to Quaternary transpressional reactivation and active mountain building in the area. Although the AFT annealing model used, may produce artefactual Late Tertiary cooling events, the Pliocene to Quaternary cooling event registered here is thought to be real since it is supported by specific AFT track length distributions aswell as by abundant field evidence. References Buslov, M.M., Watanabe, T., Smirnova L.V., Fujiwara, I., Iwata, K., De Grave, J., Semakov, N.N., Travin, A.V., Kiryanova, A.P., Kokh, D.A. (2003). Role of strike-slip faults in Late Paleozoic-Early Mesozoic tectonics and geodynamics of the Altai-Sayan and East Kazakhstan folded zone. Russian Geology and Geophysics, 44, 49-75. De Grave, J., Van den haute, P. (2002). Denudation and cooling of the Lake Teletskoye region in the Altai Mountains (South Siberia) as revealed by apatite fission-track thermochronology. Tectonophysics, 349, 145-159. Dehandschutter, B., Vysotsky, E.M., Delvaux, D., Klerkx, J., Buslov, M.M., Seleznev, V.S., De Batist, M. (2002). Structural evolution of the Teletsk graben (Russian Altai). Tectonophysics, 351, 139-167.